This invention generally relates to a process for preparing structured latex particles which are film forming and whose films exhibit high strength and/or corrosion resistance. Further, this invention is directed toward a continuous polymerization process to form structured latex particles whose films exhibit improved physical characteristics.
Aqueous dispersions of polymers, which are referred to in the art as latexes, are generally known to be useful, both alone and in various formulations, as coatings, adhesives and impregnants. A wide variety of latexes of differing homopolymeric and copolymeric composition (such as styrene/butadiene copolymers, acrylic homopolymers and copolymers, vinylidene chloride homopolymers and copolymers, etc.) have been developed having specific chemical and/or mechanical properties for particular end use applications. For example, aqueous interpolymer latexes resulting from the emulsion polymerization of monovinyl aromatic monomers, such as styrene; diolefins, such as butadiene; and monoethylenically unsaturated carboxylic acids, such as acrylic acid; are known to be particularly useful as film forming binders for pigments in paper coating applications. See, for example, U.S. Pat. Nos. 3,399,080 and 3,404,116.
Aqueous polymer latexes comprising substantial or predominant amounts of vinylidene chloride copolymerized with relatively smaller amounts of other monomers are known to possess a community of desirable properties making them well adapted for wide variety of end uses. Among such properties are reduced flammability, low permeability to oxygen and water vapor, chemical inertness including resistance to greases and oils, good binding power for pigments, fillers, etc., high impact and tensile strength, and the like.
Unfortunately, however, the foregoing advantageous properties of the known latexes are accompanied, to greater or lesser degrees, by certain disadvantageous properties including limited flexibility and elongation, colloidal instability (e.g., sensitivity to, or coagulation upon, storage and/or exposure to mechanical shear). Further, known latexes can suffer from chemical instability (e.g., degradation and/or discoloration by exposure to polyvalent metal ions), water sensitivity, heat and light instability (e.g., degradation and/or discoloration upon exposure to heat and/or light), and corrosive instability (e.g., corrosion occurs from the generation of HCl from the vinylidene chloride moiety present in the latex).
In view of the indicated disadvantages of prior art latexes, it is highly desirable to provide improved latexes which overcome some or all of such disadvantages, particularly those related to film strength (tensile amd elongation) and corrosion resistance.